Polyimides of ether-linked aryl tetracarboxylic dianhydrides

ABSTRACT

Polyimides comprised of repeating units having the structure:   WHEREIN Rf is perfluoralkylene having the structure (CF2n or a perfluoroalkylene ether having the structure (CF2)m-O-(CF2)m wherein n is an integer of 2 to 10, and m is an integer of 1 to 10; x and y are each 0 to 3 and x + y 0 to 5; and m&#39;&#39; is 1 to 100 have been found to possess improved thermal, oxidative and hydrolytic stability as well as improved physical and chemical characteristics that make it useful in a variety of applications including that of a sealant in advanced aerospace structures.

i United States Patent [1 1 Fletcher et al.

[ POLYIMIDES 0F EITHER-LINKED ARYL TETRACARBOXYLIC DIANHYDRIDES [75] inventors: James C. Hetcher, Administrator of the National Aeronautics and Space Administration, with respect to an invention of James A. Webster, Dayton. Ohio [73} Assignee: The United States of America as represented by the Administrator of the National Aeronautics and Space Administration, Washington, DC.

[22] Filed: July 11, I974 [21] Appl. N0.: 487,852

[52] US. Cl. 260/47 CP; 260/328 N; 260/32.6 N; 260/78 TF; 260/3463; 260/57l [Sl] Int. Cl. C08G 73/10 [58] Field of 260/47 CP, 78 TF, 346.2,

[56] References Cited UNITED STATES PATENTS 3/l972 Critchley et al. 260/78 TF 4/1974 Takehoshi et al 260/465 E Primary Examiner-Lester L. Lee Attorney. Agent, or Firm-Wayland H. Riggins; L. D. Wofford, Jr.; John R. Manning [57] ABSTRACT Polyimides comprised of repeating units having the Dec. 9, 1975 5 Claims, No Drawings POLYIMIDES F EITHER-LINKED ARYL TETRACARBOXYLIC DIANHYDRIDES ORIGIN OF THE INVENTION The invention described herein was made in the per formance of work under a NASA contract and is subject to the provisions of Section 3050f the National Aeronautics and Space Act of 1958, Public Law 1 85-868 (72 STAT. 345', 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to novel polyimides of improved chemical and physical properties and to a method for their preparation.

2. Description of the Prior Art A very definite need has developed for materials useful in the preparation of sealants in advanced aerospace structures. The major requirement that such sealants must satisfy is the ability to maintain a seal within a fuel tank for an extended period of time, normally up to 25,000 hours, while subjected to a variety of adverse conditions. The conditions include exposure to high and low temperatures (45C to 260C) in the presence of oxygen, hydrocarbon fuel and moisture. These conditions necessitate a high level of thermal, oxidative and hydrolytic stability. In addition, the sealant must also exhibit adequate physical and chemical characteristics including tensile strength and elasticity, good adhesion, and freedom from stress corrosion of titanium alloys. Needless to say the aforementioned requirements of sealants for aerospace are extraordinarily stringent and very few are known which meet the desired properties.

SUMMARY OF THE INVENTION One object of the invention, therefore, is to provide polyimides exhibiting excellent thermal, oxidative and hydrolytic stability and high tensile strength and elongation during exposure to temperatures ranging from 45C to 260C.

Another object of the invention is to provide novel polyimides of improved fuel resistance, lowered glass transition temperature and improved high temperature capabilities. I

Yet another object of the invention is to provide a method for the production of the novel polyimides of the invention.

These and other objects of the invention will become apparent from the following description and appended claims.

In accordance with the present invention, there are provided novel, normally solid, non-crosslinked polyimides comprised of repeating units having the structure:

rcr cro) tcr 5 (ocrc wherein Rf is perfluoroalkylene having the structure (CFQN or a Perfluoroalkylene ether having the structure (C F 2)...O (CF wherein n is an integer of 2 to 10, preferably 4 to 6 and m is an integer of l to 10, preferably 4 to 6; x and y are each 0 to 3 and x y= 0 to 5, and m is generally 1 to I00.

The polyimides of the invention have been found to exhibit an extraordinarily high level of thermal, oxida- 0 tive and hydrolytic stability. Solutions of the polyimides in organic solvents such as acetone or dimethylacetamide can be cast into films which exhibit high strength and elongation. Compression molding of the linear polymers has also been demonstrated.

If desired, a small proportion of a trifunctional crosslinking agent can be incorporated into the polyimides to impart high temperature mechanical stability and increased solvent resistance. The resulting crosslinked polyimides are found to exhibit high stability and appreciable tensile strength and elongation at temperatures as high as 300C. In addition, the polyimides of the invention are characterized by high resistance to hydrocarbon fuels which makes them highly useful in the preparations of sealants for use in advanced aerospace structures.

DETAILED DESCRIPTION OF THE INVENTION The novel polyimides of the invention may be prepared by condensation polymerization of an etherlinked aryl tetracarboxy dianhydride having the strucwherein Rf is a perfluoroalkylene having the structure {CI- or a perfluoroalkylene ether having the structure-(-CF,-)-,,,O(CF, wherein n is an integer of 2 to l0, preferably 4 to 6 and m is an integer of l to 10, preferably 4 to 6, with a diamine having the structure:

wherein x and y each to 3 and x y 0 to 5.

Preparation of the polyimide polymers may be conducted in a conventional manner (e.g. as described in W. R. Sorenson and T. W. Campbell, Preparative Methods of Polymer Chemistry", lnterscience Pub. Sec. Ed., [968, pp. I70) by the addition of the etherlinked aryl tetracarboxydianhydride to the diamine dissolved in a mutual solvent such as dimethylacetamide. The diamine and dianhydride reactants are employed in about stoichiometric proportions but small deviations from I :1 proportions show little apparent changes in physical or chemical properties. The initial reaction at temperatures near ambient is the formation of a solution of polyamic acid. in general a reaction temperature of about to 80C. is employed. Formation of the fluorocarbon polyamic acids, however, takes place more slowly than with conventional polyimides, presumably because of the lower base strengths of the fluorocarbon aromatic diamines. We should not preclude acceleration of this reaction by heating to 30-80C. An appreciable increase in solution viscosity becomes apparent only after stirring for over say 12 hours whereas with conventional polyimides a maximum viscosity is normally attained within a few hours. Evaporation of As aforementioned, if desired, a small proportion of a trifunctional crosslinking agent can be incorporated to impart high temperature mechanical stability and increased solvent resistance. Any of the conventional trifunctional crosslinking agents for polyimides can be employed. A particularly preferred crosslinking agent is mellitic trianhydride. When used the crosslinking agents are employed in amounts of about 2 to equivalents percent preferably about 5 to 15 equivalents based on the dianhydride employed in the polymerization.

The ether-linked aryl tetracarboxydianhydride reactant of the present invention may be prepared by a series of steps comprising reacting 3,4-bis(trifluoromethyl) phenol and a perfluoroalkanedioyl halide in a molar ratio of at least 2:1 to produce the corresponding 3,4-bis(trifluoromethyl)phenyl perfluoroalkanedioate. The 3,4-bis(trifluoromethyl phenyl perfluoroalkanedioate is then fluorinated to produce 3,3,4,4'-tetrakis (trifluorometh'yl)-a,m-diphenoxy polyfluoroalkane which is hydrolyzed to the corresponding tetracarboxylic acid. The tetracarboxylic acid thus produced is then dehydrated in the dianhydride whr?t and heating to about 150C to 2500C results in reactant of the invention. The synthesis of the dianhycondensation of the polyamic acid to the polyimide.

I II

dride reactant may be outlined as follows:

i? P cr c-mr- --o CFa The perfluoroalkanedioyl halide reacted with the compounds having the structure:

wherein Rf is as defined above and X is F, C1 or Br.

Examples of perfluoroalkanedioyl halides are perfluoromalonyl chloride, perfluorosuccinyl chloride, perfluoroglutaryl chloride, perfluoroadipoyl fluoride, etc. The reaction is ordinarily conducted in the presence of catalytic quantities of an aprotic amine catalyst such as pyridine at an elevated temperature, preferably about 75C to 160C using a molar ratio of 3,4-bis(trifluoromethyl) phenol to perfluoroalkanedioyl chloride of at least 2 to 1.

F luorination of compound Ill, i.e. the 3,4-bis( trifluoromethyl) phenyl perfluoroalkanedioate is effected by conventional methods known to convert carboxyl groups to CF, groups. A convenient method comprises 1 pressurizing an autoclave containing compound 111 with a stoichiometric excess of sulfur tetrafluoride in admixture with anhydrous hydrogen fluoride and heating the reactant mixture at 80C 85C.

The hydrolysis of the resulting 3,3',4,4'-tetrakis(trifluoromethyl)-a,m-diphenoxy polyfluoroalkane (Compound IV) is a preferential hydrolysis of the CF groups on the aryl ring of compound 1V without attack on a EXAMPLE 1 Preparation of Ether-linked Aryl Tetracarboxydianhydride 3 ,4-Bis(trifluoromethyl )phenol 3,4-Bis(trifluoromethyl)aniline (24 g, 0.15 mole) was dissolved in 120 ml of concentrated sulfuric acid. A solution prepared by dissolving sodium nitrite (12.0 g, 0.174 mole) in 120 m1 of cold concentrated sulfuric acid was added to the amine over a period of 75 minutes. The temperature was then permitted to rise to 25C and stirring was continued for 2 hours. The reaction mixture was then poured over excess ice and the resulting aqueous solution was steam distilled until 2 liters of distillate was collected. Extraction of this aqueous solution with ether and distillation of the extract afforded 19.3 g (80% yield) of 3,4-bis(trifluoromethyl)pheno1, bp 116C/50 torr, 1 1.4286.

3 ,4-Bis(trifluoromethy1)phenol Hexafluoroglutarate Pyridine catalyzed reaction of 3,4.-bis(trifluoromethy1)phenol (7.8 g, 0.034 mole) with perfluoroglutaryl chloride (4.7 g, 0.017 mole) at 75 to 120C formed the corresponding diester in 90% yield, hp 155C/0.15 torr, 1 1.412348.

3,3 ",4,4'-Tetrakis(trifluoromethyl)-l ,5'diphenoxydecafluorpentane A 300 stainless steel autoclave was charged with 8.0 g (0.012 mole) of 3,4-bis(trifluoromethyl)pheno1hexafluoroglutarate, 38 g (1.9 moles) hydrogen fluoride and 24 g (0.22 mole) sulfur tetrafluoride. The autoclave was heated for 3 hr at C and 21 hr at C. The pressure was released after the autoclave had cooled and the contents was poured over ice and neutralized with Nal'iCO The product was extracted with chloroform, washed with NaHCO,. The product was extracted with chloroform, washed and dried. Distillation gave 8.5 g of product considered to be 3,3'4,4-tetrakis( trifluoromethyl l ,5-diphenoxydecafluoropentane, bp 115, 122Cl0.15 torr, 1 1.3895. Infrared analysis showed no carbonyl absorption.

1,5-Diphenoxydecafluoropentane-3 ',3 ',4',4 '-tetracarboxylic Acid A mixture of 3,3",4',4"-tetrakis(trifluoromethyl)- l,5-diphenoxy-decafluoropentane (28.5, 0.049 mole), 100% sulfuric acid (25 g, 0.25 mole), and chlorosulfonic acid (29 g, 0.25 mole) was heated with stirring in a flask under a condenser with dry nitrogen atmosphere. The temperature was held at C for 48 hours, C for 16 hours, and C .for 24 hours. During this time HCl evolved slowly. The mixture was then poured over ice, extracted with ether, washed and dried. Evaporation of ether left a brown gummy solid. This was dissolved in 300 ml water at 85C, decolorized with charcoal, and filtered hot. Upon cooling, 19 g of white solid was collected by filtration, neut. equiv. found 160; calcd. for tetracarboxylic acid, 153.

This product was refluxed with excess acetic anhydride and then devolatilized. Sublimation of the solid residue at 160C/ 0.01 torr afforded 17.3 g of dianhydride, mp 9497C (60% yield).

Attempts to purify the dianhydride further by recrystallization failed for lack of a suitable solvent. The anhydride (17.3) was finally dissolved in water and recrystallized as the acid, mp -l55C, neut. equiv. 159. The acid was then recrystallized from 27% (by volume) acetic acid/water solution and a second time from 35% acetic acid solution. The melting point was raised to l5,l153 with very little loss in material. Neut. equiv. found 159, calc'd/ 153. NMR analysis showed an equal proportion of labile and aromatic protons suggesting that the t'etracarboxylic acid crystallized as the monohydrate with a calculated equivalent weight of 157.5.

The above tetraacid was then converted once again to dianhydride and sublimed. Three fractions of sublimed material were collected.

Weight mp Neut. Equiv.

g C Found Calc'd.

wherein it andyeach=0to3 and x+y=0to 5.

The polyamides prepared by the process of the inven- F tion have a variety of applications such as fuel tank 5 sealants, matrix resins for composites, molding resins,

films, etc.

The following examples are included to further illus' trate the present invention.

EXAMPLE II Analysis for C..H.Fm

Cllcd 43.77 L05 32.97 Found 43.53 0.96 33.")

Preparation of Diamine m The diamine reactant of the invention may be prepared by the SF. fluorination of the nitrophenyl ester of a perfluoroalkylene ether dicarboxylic acid, followed l,5 Diphenoxydecafluoropentane-3'3",4'4"'- by catalytic reduction of the nitro groups at 40 psi hyracarboxylic acid dianhydride (0.144 g, 25 millimole) drogen over Raney nickel to form the diamine. The was added to (0.24] g, 25 millimole) of adiamine havsynthesis may be represented as follows: ing the following structure:

\ r: H 11 lar e m (C9 5 oc-cl rzi' 00/ Preparation of Polyimide CFO wherein x y 3 dissolved in 2 ml dimethylacetamide. The mixture was allowed to stir overnight (19 hr). The resulting viscous solution was poured into a film mold and warmed at 60 to 80C to facilitate evaporation of solvent. The film of polyamic acid was then heated gradually (over several hours) to a temperature of 200C. The resulting polyimide film exhibited high strength and elongation.

EXAMPLE Ill Three different polyimide preparations designated A, B and C in the Table 1 below were prepared using the general procedure described in Example ll except that about equivalents of the ether-linked dianhydride was replaced with mellitic trianhydride as a crosslinking agent. The polyamic acid solutions A & B were stirred l hour at C before being poured into the film mold. Polyimide C contained a 2% excess of dianhy- The tensile strength measurements carried out using microtensile test specimens approximate but may differ from values obtained using standard tensile test specimens. The reported elongation values are based on crosshead travel because of the inability to use an extensiometer. If the length of the necked-down portion of the die is considered in gauge length, the results will be too high when appreciable elongation occurs within the entire length of the specimen, between the grips. On the other hand, inspection of elongated, cold drawn specimens showed that the elongation of some specimens occurred primarily within the necked-down portion. Because of this, maximum and minimum elongation values are shown in the Table. The maximum elongation results are based on the 17 mm gauge length, and minimum values are those based on 23 mm, the specimen length between grips. The results of the tests are summarized in the following Table I.

Table TENSILE STRENGTH AND ELONGATION OF POLYIMIDE x v Y=3 Composition (Equiv. Fraction of Reactants) Amine Mellitic COMPOSITION Anhydride, R O(CF -,),O x y 3 Trianhydride A 0.9 1.0 0.1 B 0.9 1.0 0.1 C 0.92 1.0 0.1

Cure Cure Test Tensile I: Elongation Temp. Time Temp. Strength, (23 mm 17 mm COM POSITION C hr. C psi gauge) gauge) A 200 4 -43 5900 25 5200 230 320 288' 165 70 B 200 l 43 7000 25* 8500 290 400 288 190 50 65 C I90 66 43 7300 130 I80 25" 6100 300 420 288' 130 S0 70 l'wo specimens Three specimens dride over and above that required by stoichiometry. Each of the polyimides was subjected to cure by gradually raising the temperature to 200C followed by an additional hour or more at 200C.

The polyamides A, B and C thus prepared were subjected to tensile strength and elongation measurements which were determined using microtensile test specimens. The specimens were cut with the aid of a nonstandard die, similar in shape but smaller than that designated by ASTM procedure D-412-64l. The die dimensions, designated by the ASTM procedure as A, C, L, and W, were 8, 44, 17 and 2.6 mm. respectively.

EXAMPLE lV The polyimide C was subjected to Chevron A-50 jet fuel under the conditions shown in Table 11 and the tensile strengths and elongations were determined before and after exposure. The results are set forth in Table ll.

1 1 l2 Polymer C exposed to fuel vapor at atmospheric It is claimed: pressure for 200 hours at 288C showed a significant l. A normally-solid polyimide consisting essentially increase in elongation with a moderate decrease in tenof repeating units having the structure:

. P-(rm-o H 13 i3 c Q [l o sile strength. Similar properties were observed after a wherein Rf is perfluoroalkylene having the structure specimen of the polyimide was refluxed in fuel for 64 15 +(CF,),,+ or a perfluoroalkylene ether having the hours at I80C while exposed to air. structure -(CF,),.-O-(CF,),,- wherein n is an integer of 2 to 10 and m is an integer of l to 10;): and y are each EXAMPLE v 0 to 3 and x y 0 to 5; and m is l to 100.

The adhesion of films of the polyimides of Example The polyimide of ciaim 1 wherein Rfi ill to stainless steel and titanium metal were investiand n is 4 to gated. The metal specimens were cleaned and then coated with the polyimide polymer composition and cured at l80C. The strips were then refluxed for 70 hours in Chevron A-SO jet fuel at 180C under a nitrogen atmosphere for one example which was carried out in contact with air.

The results were good adhesion and resistance to refluxing jet engine fuel. 4: s is =0: 1

3. The polyimide of claim 2 wherein n is s.

4. The polyimide of claim 1 wherein the sum of x y is 3.

5. The polyimide of claim I crosslinked by reacting with mellitic trianhydride. 

1. A NORMALLY-SOLID POLYIMIDE CONSISTING ESSENTIALLY OF REPEATING UNITS HAVING THE STRUCTURE:
 2. The polyimide of claim 1 wherein Rf is -(CF2)n- and n is 4 to
 6. 3. The polyimide of claim 2 wherein n is
 5. 4. The polyimide of claim 1 wherein the sum of x + y is
 3. 5. The polyimide of claim 1 crosslinked by reacting with mellitic trianhydride.
 100. 